I think my PIP backpay is too low.

I applied for a higher rate of PIP in February, had the reassessment in around June/July, at which they awarded me zero points and stopped my PIP altogether.  I appealed and won, and in November got a letter saying that I was getting the highest amount of PIP.  I got about £2326 backpay, but it seems too low.  I am no mathematician, and do please point out if I've made a glaring error.

So, from February, while waiting for my assessment, my PIP payments carried on being paid at £60 a week.  The letter says I should have been getting a total of £152 a week.  So minus the £60 a week I was getting, the backpay from February to July should be 
152 - 60 = 92 a week
= 368 a month
= 1840 for 5 months (up to and including July)

My last PIP payment before it was stopped was July, so I should get backpay for the full amount for Aug, Sept and Oct:
152 x 12 weeks = 1824

1840 + 1824 = 3664

I only got £2326 backpay.  it seems to me they should have given me another £1338.  Am I wrong?